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Question: PaK 1935 has been researched but does not appear for selection in the production window for brigades! Where is the problem?

The anti-tank gun? There's no brigade for AT guns in DH. Researching the tech increases the hard attack of ART, INF, and MTN
 
Assuming you are talking about anti-tank artillery 1935 DH Full, you only get improved hard attack for artillery, inf and mtn; and lose some soft attack for art.

You don't get a AT brigade if that's what you are looking for.

Edit: Read what the tech does